Milionaire Lionel Twain invites five famous private eyes to a mysterious dinner at a remote castle. The wise Sidney Wang, the urbane couple Dick and Dora Charleston, the canny Englishwoman Jessica Marbles, the clever Milo Perrier from Belgium, and tough-talking Sam Diamond all arrive wondering why they have been invited. When the private eyes and their guests sit down to dinner, they meet Mr.Twain who explains that a murder will take place at midnight, and that whoever can solve the crime will win a million dollars....